The electroless plating of metals onto copper is becoming increasingly valuable to the semiconductor industry, for logic, memory, and photovoltaic devices. As the scale of the copper features are scaled down, the use of electroless deposition has become valuable because of its conformal nature and its ability to deposit thin films selectively on complex and small scale features.
For logic devices, metals such as cobalt and its many alloys have been used to form capping layers over copper interconnect lines to prevent electromigration of the copper lines. But, cobalt or alloys of cobalt tend to oxidize and may increase the effective resistance of the copper lines. Metals that will not oxidize, such as noble metals, can be used in place of cobalt but are difficult to plate onto the increasingly small copper features. Either galvanic initiation by aluminum contact or thick copper (greater than 300 angstroms thickness) has to be used to obtain platinum film deposition. Thick copper films have been necessary for the plating of platinum onto copper because the activation step attacks and degrades the copper.
Electroless deposition may also have valuable applications for memory devices. This is because the blanket deposition of a platinum film on copper by physical vapor deposition (PVD) requires multiple steps to etch the platinum from the regions where it is not desired (e.g. over the dielectric layer.) Plasma etching of platinum is very difficult to do in production because of poor pattern definition due to the primarily physical sputtering and high tool maintenance needed after heavy platinum deposition on the chamber walls. The deposition of platinum on the deposition chamber walls requires constant cleaning to maintain process stability.